A nuclear medicine diagnosis apparatus performs, for example, PET (positron emission tomography) acquisition in the following manner. First of all, a medicine labeled with a radioactive isotope which emits positrons is administered to a subject. The nuclear medicine diagnosis apparatus repeatedly detects gamma rays emitted from the subject by using a plurality of photodetectors arranged around the subject in a ring form. The nuclear medicine diagnosis apparatus uses the detection times of gamma rays as time stamps, and identifies two gamma rays detected within a predetermined time frame. The two identified gamma rays are estimated to be generated from the same pair annihilation point. The nuclear medicine diagnosis apparatus estimates that the pair annihilation point exists on a line (LOR: line of response) connecting a pair of detectors by which the gamma rays have been simultaneously measured. To identify two gamma rays generated from the same pair annihilation point in this manner is called coincidence. The nuclear medicine diagnosis apparatus generates a PET image data based on output signals from the photodetectors associated with the LOR.
In order to specify the emission position of a gamma ray propagating at the velocity of light (about 300,000 km/s), it is necessary to assign the gamma event an accurate time stamp on the order of 10 ps. The nuclear medicine diagnosis apparatus is required to have a very high time resolution. This requires high-accuracy clock synchronization for all the photodetectors. This demands a very complicated and expensive mechanism. Furthermore, it is technically very difficult to implement clock synchronization on the order of 10 ps.
Each photodetector has an intrinsic response time (rise time). For this reason, the response time of each photodetector is measured in advance. The response times of the respective photodetectors are then corrected in accordance with the measurement results (calibration data) so as to equalize the response times of all the photodetectors. This correction is called timing calibration. The time resolution of the nuclear medicine diagnosis apparatus is improved by correcting the response times of all the photodetectors so as to equalize them. However, a very long time is required to obtain calibration data about all the photodetectors. In addition, the response time of each photodetector changes over time. For this reason, obtained calibration data is effective for only a short period of time. It is therefore necessary to frequently perform timing calibration for a long period of time. This imposes a very heavy load on the user.